Worldwide social network ad revenues will reach $5.54 billion this year, eMarketer estimates, with just under half that amount, $2.74 billion, coming from the US market.

Revenue growth is solidly in the double digits in the US, but even more rapid growth elsewhere will mean spending outside the country will account for a slightly greater share each year. By 2013, non-US revenues will make up 51.9% of the total, which will hit nearly $10 billion worldwide. In the US, social networks will make $4.81 billion from ads that year.

The bulk of these dollars, in the US and around the world, will go to Facebook, while a much smaller share will go to Twitter and other social networks. eMarketer's first forecast of ad revenues for LinkedIn predicts the site will account for 3% of worldwide social network ad revenues this year, with $140.8 million. The site has more than tripled its ad dollars in two years, though growth is tapering off.

Gains in social network ad revenues mean these sites account for an ever greater share of all digital ad spending. This year, 8.8% of online ad dollars in the US and 6.9% worldwide will go to social networking sites, eMarketer estimates. By 2013, social network ad revenues will make up 11.7% of all online ad spending in the US and 9.4% around the world.

eMarketer forms its estimates of social network ad revenues based on a meta-analysis of estimates of consumer usage, marketer usage, ad pricing and impressions, as well as revenue estimates from research firms and other sources, and from interviews with industry executives.